If you planning on buying more memory too I would just rebuild with amd. You can get the Athlon II x4 630
, and a Decent Asus Mobo
for what you would pay for the comparable core 2 q9550. Plus you can buy ddr3 memory at the same price as ddr2 nowadays. This would give you better upgrade options down the road.
for the price of your upgrade 4Gb ddr2 + 775 socket CPU will cost alomost the same as a nice AMD combo (CPU+DDR3+MB).
for the price of the CPU upgrade only you can get a AMD cpu+mb combo.